The present invention generally relates to systems and methods for non-destructive testing, and relates in particular to non-destructive testing systems and methods for use in testing fiber-reinforced polymer composite jacketing systems.
Fiber-reinforced polymer (FRP) composite jacketing systems have emerged as an alternative to traditional construction, strengthening, and repair of reinforced concrete columns and bridge piers. A large number of projects, both public and private, have used this technology and escalating deployment is expected, especially in seismically active regions.
Existing damages, however, in the interface vicinity, debonding of FRP from concrete, debonding between layers of a FRP wrap and those in overlap joints, may lead to catastrophic failures at loading stages earlier than those corresponding to intact specimens. Also, overlap joint length reduction due to progressive debonding during a seismic event may also be possible. Overlap joint failures have been reported in laboratory tests regardless of FRP configuration of the jacketing system. All of these failures may be associated with the onset of near-surface debonding within FRP layers or between FRP and concrete. Thus, near-surface debonding is used as a precursor for the damage detection in FRP-wrapped concrete columns.
Existing evaluation methods such as visual inspection and sample extraction/testing are labor intensive, time consuming, destructive to structures, and most importantly, unable to provide sufficient information about the extent of damage in structural members confined by FRP jackets. Concrete core conditions cannot be fully revealed until physical removal of the jacketing system unless the member has already been subjected to substantive damage. Partial or complete removal of the jacket may, however, pose a danger of structural collapse. A concrete column could appear safe without showing any sign of damage on the jacket and yet containing a substantial cracked or crumbled concrete core. Such scenario could happen when the structure has undergone a modest seismic event damaging the FRP/concrete system while not causing the ultimate failure of the system. That column may not live up to a second seismic event, however, because of the reduced resistance due to the existing invisible damage in the concrete and in concrete/FRP interface region.
To effectively detect and characterize concrete anomalies and FRP delamination, a NDE technique that is capable of detecting (1) the extent of concrete cracking, crumbling, and FRP delamination from concrete, (2) jacket debonding in the FRP-FRP interface, and (3) sizeable air pockets trapped between FRP and concrete during manufacturing is necessary. Currently, several NDE techniques have been under investigation. They include stress wave (acoustic), infrared thermography, x-ray, and radar (microwave) techniques. Acoustic, infrared thermography, and radar techniques have recently been of particular interest to researchers for possible damage assessment of reinforced concrete and FRP-bonded concrete structures in laboratory settings. In spite of studies of such techniques, there is no currently available technology capable of visualizing and characterizing various forms of FRP-bonded concrete damages that is ready for industrial applications.
Acoustic methods are based upon elastic wave propagation in solids. They include pulse-echo, impact-echo, ultrasonic, acoustic emission, and spectral analysis of surface waves (SASW) techniques. Disadvantages include the need of intimate contact between the equipment and subject, the use of sound compliant, as well as the existence of multiple paths through the same subject that make result interpretations difficult.
Infrared thermography is based on the detection of heat flow in the subject in which air gaps resulted from delamination act as insulators, which block out the proper heat flow. Data interpretation is, however, complicated because of varying ambient temperature conditions and surface emissivity variations, which is a function of surface properties.
Radiography Radiography-based evaluation methods use high frequency electromagnetic radiation (X-rays and Gamma rays) or particular beams (beta rays and neutron radiation) passing through the subject and exposing it onto a film on the other side of the subject. Limitations include the need to access both sides of the subject, the need of safety precautions, long exposure, and two-dimensional (2D) images of three-dimensional (3D) subjects.
Microwave and radar has been used for site characterization in geotechnical engineering, and have also been used to evaluate concrete structures, pavements, and bridge decks. Radar involves the generation and transmission of electromagnetic waves into materials such as concrete with different dielectric constants. Voids, delamination, rebars, and material characteristics can be detected and interpreted from the reflected waves. Optimization between penetration depths and detection capability, two inversely related parameters that are dependent on the frequencies and bandwidth of the wave, could be a challenge. Conventional radar typically makes use of low frequencies to enhance penetration but with sacrificed detectability.
There is a need, therefore, for a system and method for detecting damage, defects and reinforcement conditions in fiber reinforced concrete systems that avoids the above shortcomings.